1. Field of the Invention
The invention relates to a hydraulic drive, preferably for a hydraulic press having a first synchronized cylinder that includes a first and a second pressure chamber and a piston that separates the first from the second pressure chamber and having at least one hydraulic pump that has a pump inlet and a pump outlet.
2. Description of the Related Art
Hydraulic drives of this type are well known from the current state of the art. In practice it is desirable for hydraulic drives, in particular for hydraulic drives for hydraulic presses, to provide a hydraulic drive that on the one hand provides a rapid movement of a drive piston in a so-called rapid stroke or rapid movement, and on the other hand a slower movement with great force is possible in a so-called load stroke or load movement.
Various drives are knows for this purpose from the current state of the art. In one drive with a so-called throttle control a pump is driven by a motor at constant speed. The control and changeover between rapid stroke and load stroke through control of the volume flow occurs hereby via flow resistance, for example valves. A disadvantage of such a drive with throttle control is the low efficiency due to the occurring flow losses.
Drives having a so-called displacement control system are moreover known from the current state of the art. A drive of this type may for example include a variable speed motor that drives two pumps having opposite delivery directions. The two pumps are connected with a hydraulic cylinder in such a way that the pump takes in hydraulic oil from one piston chamber of one hydraulic cylinder, whereas it moves hydraulic oil into the other piston chamber. The changeover from rapid stroke to load stroke, or respectively the speed control of the hydraulic drive, occurs through changing of the displacement volume of the pump or respectively through the change in speed of the motor. A disadvantage of such a drive with displacement control is that the motor must have a high maximum speed for the high speed in the rapid stroke, whereas a high maximum torque is required for the high force in the load stroke mode. Because of this high so-called peak performance the motor becomes large, heavy, slow and expensive.
What is needed in the art is a hydraulic drive that can be operated in a rapid stroke and a load stroke, whereby efficiency losses are avoided and whereby the motor should be able to be produced cost effectively.